The objective of this research is to study the mechanisms of somatic changes in human ring chromosomes that either occur spontaneously or are induced by mutagenic agents such as mitomycin C, X-irradiation and ultraviolet irradiation, viruses and BrdU. Methods will involve the study of such changes by chromosome banding techniques and by the BrdU-Hoechst technique for detecting sister chromatid exchanges. Aims of the study are to learn more about (1) ring chromosome stability; (2) general chromosome structure and organization; (3) effects of mutagens on chromosomes and possible test(s) for assessing mutagenic effects using ring chromosome cell lines; (4) the prognosis and risks for human patients carrying a ring chromosome.